The first time Castiel does it, it’s an accident.
He’s traveling through the Andromeda galaxy during what Earth, or, at least, the parts that follow a eurocentric Gregorian calendar, has classified as the seventeenth century. 1652, to be exact. He shuttles through the nothingness and reaches a glittering tendril of himself out, shines so bright that he dampens the light of the stars.
And then a jolt shoots through him. It’s a complicated feeling, as if everything around him and in him is bending, wobbling, coming together at once and then blowing apart. He isn’t sure whether the feeling is uncomfortable or not, and it’s too brief to let him decide.
He twists to gaze behind him. There’s a faint trace of something—a glow that’s fading away. Just barely visible.
He puts out a wavelength of himself, runs the peaks and valleys over that light. It’s familiar—it’s him, he realizes, and the soft caress of his curves over his past (or maybe future) self’s lengths sends a low humming through his entire body.
He decides that he likes it.